1. Field of the Invention
The present invention relates to a separate-function type of electrophotographic photosensitive member that has at least a charge generation layer and a charge transport layer. Particularly the invention is directed to an electrophotographic photosensitive member improved in repeated-operational characteristics, in other words, performance characteristics for continuously repeated service.
2. Description of the Prior Art
Practical use of electrophotographic photosensitive layers comprising organic photoconductive material is difficult because it is less sensitive than selenium or cadmium sulfide, but has been realized recently by achieving a high sensitivity with a separate-functional type of photosensitive layer that is a charge generation layer-charge transport layer laminate.
Charge-transporting materials in use for the charge transport layers are, for example, hydrazone compounds as described in U.S. Pat. Nos. 4,150,987 and 4,391,889 and U.K. Patent Application Publication No. 2,034,493, pyrazoline compounds as described in U.S. Pat. No. 3,824,099, and styrylanthracene compounds. Charge transport layers are formed by applying charge-transporting materials dissolved in a resin solution, since charge transporting materials are generally low molecular compounds deficient in film-forming property. Such resins include, for example, polycarbonate, polymethacrylate, polyarylate, polystyrene, polyester, polysulfone, styrene-acrylonitrile copolymer, and styrene-methyl methacrylate copolymer.
Of these resin materials, those radical-polymerized in solution contain large amounts of unreacted monomer, polymerization initiator, etc. even after completion of the polymerization. Polymer components of very low molecular weights like oligomers are also contained in such resin materials. It has been found that, when a charge transport layer is formed by using a resin which contains such non-macromolecular components as mentioned above, these components have unfavorable effects on electrophotographic characteristics of the resulting photosensitive member. According to experiments conducted by the present inventors, the above components tend to cause, in particular, phenomena such as instability of the potential, deterioration of the sensitivity, and increased optical hysteresis, i.e. increase in so-called photomemory, during repeated operations of the photosensitive member.